Ions are commonly implanted into a substrate in ion implantation processes to produce semiconductor devices. These ion implantations may be achieved in a number of different ways. For example, a beam-line ion implantation system may be used to perform the ion implantation process. In the beam-line ion implantation system, an ion source is used to generate ions, which are manipulated in a beam-like state, and then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
In another example, a plasma containing ions may be generated near the substrate. A voltage is then applied to the substrate to attract ions toward the substrate. This technique is known as plasma doping (“PLAD”) or plasma immersion ion implantation (“PIII”) process. FIG. 1 shows an exemplary plasma doping system 100. The plasma doping system 100 includes a process chamber 102 defining an enclosed volume 103. Within the volume 103 of the process chamber 102, a platen 134 and a workpiece 138, which is supported by the platen 134, may be positioned.
A gas source 104 provides a dopant gas to the interior volume 103 of the process chamber 102 through the mass flow controller 106. A gas baffle 170 is positioned in the process chamber 102 to deflect the flow of gas from the gas source 104.
The process chamber 102 may also have a chamber top 118 having a dielectric section extending in a generally horizontal direction and another dielectric section extending in a generally vertical direction.
The plasma doping system may further include a plasma source 101 configured to generate a plasma 140 within the process chamber 102. The source 101 may include a RF power source 150 to supply RF power to either one or both of the planar antenna 126 and the helical antenna 146 to generate the plasma 140. The RF source 150 may be coupled to the antennas 126, 146 by an impedance matching network 152 that matches the output impedance of the RF source 150 to the impedance of the RF antennas 126, 146 in order to maximize the power transferred from the RF source 150 to the RF antennas 126, 146.
The plasma doping system 100 also may include a bias power supply 148 electrically coupled to the platen 134. The bias power supply 148 may provide a continuous or a pulsed platen signal having pulse ON and OFF time periods to bias the workpiece 138. In the process, the ions may be accelerated toward the workpiece 138. The bias power supply 148 may be a DC or an RF power supply.
In operation, the gas source 104 supplies a dopant gas containing a desired dopant species to the chamber 102. To generate the plasma 140, the RF source 150 resonates RF currents in at least one of the RF antennas 126, 146 to produce an oscillating magnetic field. The oscillating magnetic field induces RF currents into the process chamber 102. The RF currents in the process chamber 102 excite and ionize the primary dopant gas to generate the plasma 140.
The bias power supply 148 provides a pulsed platen signal to bias the platen 134 and, hence, the workpiece 138 to accelerate ions from the plasma 140 toward the workpiece 138. The frequency of the pulsed platen signal and/or the duty cycle of the pulses may be selected to provide a desired dose rate.
The above technique is known to provide high implant throughput. However, the uniformity of the dose is difficult to control. In the beam-line ion implantation system, components such mass analyzer magnets, deceleration electrodes and other beam-line components may be used to manipulate ions into a uniform ion beam, and the workpiece may be uniformly implanted with ions in the uniform ion beam. Such components, however, are not available with a plasma doping system. To uniformly implant the workpiece in the plasma doping system, the plasma generated near the substrate should be uniform, as PLAD implant uniformity is closely related to plasma uniformity.
In a typical plasma based system, the generated plasma is typically non-uniform; the plasma density is typically higher in the center of the plasma than near the chamber walls, as shown in FIG. 4. As a result, implant profile on the workpiece shows a similar non-uniform profile—higher implant dose in the middle, and lower dose in the edges of the workpiece. Typically, RF power, gas flow and distribution, magnetic confinements, etc. may be adjusted to improve the plasma uniformity. However, such techniques may mitigate the plasma non-uniformity, but cannot change the generic non-uniform density profile shown in FIG. 4.
As such, systems and methods to improve the uniformity of the plasma in a plasma based system are needed.